Findings from observational studies by our group and others suggest that vitamin D insufficiency (a common condition) is associated with an increased risk of severe asthma exacerbations in school-aged children. We hypothesize that vitamin D supplementation reduces the incidence of severe asthma exacerbations in high-risk (so defined due to an exacerbation in the previous year) children aged 6 to 14 years who have vitamin D insufficiency (a serum 25(OH) D <30 ng/ml) and who are being treated with inhaled corticosteroids (ICS) for mild to moderate persistent asthma. We further hypothesize that this protective effect results from a reduced incidence of viral infections that lead to severe asthma exacerbations and/or enhanced response to ICS in these children. We will test this hypothesis in a randomized double-masked placebo-controlled clinical trial with the following primary and secondary Specific Aims: Primary Specific Aim 1) To determine whether vitamin D3 supplementation (4,000 IU/day) is superior to placebo in preventing severe asthma exacerbations in high-risk school-aged children who have vitamin D insufficiency and who are on ICS for mild to moderate persistent asthma. Secondary Specific Aims In high-risk school-aged children with vitamin D insufficiency who are on ICS for asthma, to determine whether vitamin D3 supplementation is superior to placebo in: 1) Preventing severe asthma exacerbations resulting from viral infections 2) Reducing the daily dose of ICS as well as the average cumulative dose of ICS, at the end of the trial.